Re: typeof and operands in named address spaces

[Nick Desaulniers <ndesaulniers@google.com>]

On Mon, Nov 9, 2020 at 11:46 AM Segher Boessenkool
<segher@kernel.crashing.org> wrote:
>
> On Mon, Nov 09, 2020 at 01:47:13PM +0100, Peter Zijlstra wrote:
> >
> > + lots of people and linux-toolchains
> >
> > On Wed, Nov 04, 2020 at 07:31:42PM +0100, Uros Bizjak wrote:
> > > Hello!
> > >
> > > I was looking at the recent linux patch series [1] where segment
> > > qualifiers (named address spaces) were introduced to handle percpu
> > > variables. In the patch [2], the author mentions that:
> > >
> > > --q--
> > > Unfortunately, gcc does not provide a way to remove segment
> > > qualifiers, which is needed to use typeof() to create local instances
> > > of the per-cpu variable. For this reason, do not use the segment
> > > qualifier for per-cpu variables, and do casting using the segment
> > > qualifier instead.
> > > --/q--
> >
> > C in general does not provide means to strip qualifiers.
>
> Most ways you can try to use the result are undefined behaviour, even.

Yes, removing `const` from a `const` declared variable (via cast) then
expecting to use the result is a great way to have clang omit the use
from the final program.  This has bitten us in the past getting MIPS
support up and running, and one of the MTK gfx drivers.
-- 
Thanks,
~Nick Desaulniers